i personally believe that past lives do exist, you only have to listen to young children to know they have been here before, but on the evidence side, there are some brilliant past life regression tapes that have been veryfied and i personally love to watch or hear about them.
as to why we would have past lives im on the fence with my thoughts as i have heard many theories on the subject that seem to make a lot of sense and i havent decided which makes the most sense yet :)